The Species That Broke Evolution?

This video is sponsored by Skillshare.  More about them at the end of the video. One hundred million years ago, the  ancestors of humans looked like this.

At the very same time in  history, the ancestors of gars, horseshoe crabs and coelacanths  looked like this, this, and this. Darwin called species like these  “living fossils'' because they seem like they are evolutionarily  frozen in time. But Darwin was wrong.

Hi, I’m David and this is MinuteEarth. All species change over time: it’s  a biological rule –mostly due to how genes work. As individuals  reproduce, DNA randomly mutates.

Some of these mutations hurt  the organism's ability to survive and reproduce, so they don’t make the cut. But others help – or at least  don’t hurt – so they stick around. Over time, mutations pile up, changing species  in all sorts of ways.

That’s evolution. But evolution doesn't happen at the same rate  in all species. Predatory fish known as gars have genes that mutate thousands of times  slower than those of most other vertebrates, although scientists don’t  totally understand why that is.

So despite what Darwin thought, gars are changing,  they’re just changing really really slowly. In fact, even though the alligator gar  and the longnose gar last shared a common ancestor more than 100 million years ago,  these fish evolve so slowly – and thus are still so genetically similar – that they  can still mate and make fertile offspring. That’s like if Chimpanzees and Kangaroos  – who also shared a common ancestor around that time – were able to make a baby.

For  the record – that can’t actually happen. But not all so-called living  fossils evolve slowly. Coelacanths and horseshoe crabs  have genes that mutate at pretty much the same rate as those of most other animals.

And the tuatara – another so-called living  fossil – actually has the fastest evolving genes of any vertebrate studied. So why don't  these animals LOOK like they're changing? For the horseshoe crab, it seems that  its particular body plan – you know, the classic domed shell and rudder-like  tail – just works super well in its marine environment – and has for a long time.

It might be that every time mutations  significantly change this body plan, the crabs become slightly-less-well-suited to their environment, and future mutations  end up bringing that successful form back. The coelacanth, meanwhile, actually has been  evolving in ways that do change its appearance; it's just that the changes are so subtle  that most of us don’t notice them. Like, if you look carefully at the skulls  of a modern coelacanth and its ancestor, you can see that – despite their incredible similarity – their facial bones are  actually in very different places.

As for the tuatara, we're not totally  sure why it’ s evolving so quickly, but it seems like both things are happening –  its skeleton is subtly changing over time AND its particular body shape seems to be almost perfectly  adapted for its life in and out of a burrow. And finally, a species could be changing in ways  that we just can't see in the fossil record. For example, we know that some ancient  coelacanth relatives lived in freshwater, so modern ones must have evolved in ways that  helped them deal with excess salt and buoyancy.

So there’s no such thing as a living fossil. Life on Earth is constantly evolving. It’s  just that there are lots of ways to do that, and for some species, the more things  change, the more they stay the same.

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